Re-configurable optical add-drop multiplexer

ABSTRACT

A re-configurable optical add-drop multiplexer (OADM) ( 800 ) includes a first optical circulator ( 10 ), a second optical circulator ( 30 ) and a fiber Bragg grating (FBG) ( 20 ). The first optical circulator has a first circulator port ( 11 ), a second circulator port ( 12 ) and a third circulator port ( 13 ). The second optical circulator has the same structure as the first optical circulator. The FBG has a first state where it reflects an optical signal with a particular wavelength and passes all other wavelengths and a second state where all the optical signals pass through it. Signals of the particular wavelength can thus be dropped from the transmitted signals by reflection of the FBG and output from the third circulator port of the first optical circulator, and new signals of the same dropped wavelength can be added into the transmitted signals though a first circulator port ( 31 ) of the second optical circulator.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical add-drop multiplexer(OADM), and particularly to a re-configurable OADM.

[0003] 2. Description of Related Art

[0004] Wavelength division multiplexing (WDM) is widely used in opticalcommunication systems. There is a need to route one or more channels ofa WDM signal to different destinations. Thus, the optical add-dropmultiplexer (OADM) is introduced into the optical communication systemto drop optical signals having the desired wavelengths from an opticalWDM signal and to add optical signals having the same wavelengths andcarrying new information. The OADM also needs to be re-configurable tobe flexibly accommodated in various applications.

[0005] U.S. Pat. No. 6,035,080 discloses a re-configurable OADMincluding at least one re-configurable add-drop unit that can add-dropone channel out of a large set by mechanically switching the light paththrough one of a set of fixed add-drop filters. Re-configuration is doneby switching from the add-drop filter path to a bypass path, changing toa different add-drop filter and then switching back. However, only oneof the set of fixed add-drop filters can be used to add-drop onechannel, so this re-configurable OADM is not cost-efficient. Inaddition, the number of channels which can be dropped by thisre-configurable OADM is unchangeable.

[0006] Therefore, an improved re-configurable OADM is required toovercome the above problems.

SUMMARY OF THE INVENTION

[0007] Accordingly, an object of the present invention is to provide are-configurable which is cost-efficient.

[0008] Another object of the present invention is to provide are-configurable OADM wherein the number of the dropped channels ischangeable.

[0009] In order to achieve the objects set forth above, are-configurable OADM according to the present invention comprises afirst optical circulator, a second optical circulator and a fiber Bragggrating (FBG). The first optical circulator has a first circulator port,a second circulator port and a third circulator port. An optical signalpassing into the first circulator port is routed to the secondcirculator port and an optical signal passing into the second circulatorport is routed to the third circulator port. The second opticalcirculator has a first circulator port, a second circulator port and athird circulator port, and the second optical circulator has the samestructure as that of the first optical circulator. The FBG has anoptical fiber, a cooler and a plurality of heating elements. The opticalfiber has an optical fiber core being made of a material whose index ofrefraction varies with temperature. The heating elements are evenlyarranged on the optical fiber, separated from one another by a fixeddistance, and the cooler is mounted on the optical fiber to maintainsections of the optical fiber between adjacent heating elements at apre-determined temperature. The FBG has a first state in which itreflects an optical signal with a particular wavelength and a secondstate in which all the optical signals pass through the FBG. The FBG isconnected with the second circulator port of the first opticalcirculator and with the second circulator port of the second opticalcirculator. At time when the FBG is selected to be in the first state,an optical signal can be added to the optical WDM signal through thefirst port of the second optical circulator and an optical signal can bedropped from the optical WDM signal through the third circulator port ofthe first optical circulator.

[0010] Other objects, advantages and novel features of the inventionwill become more apparent from the following detailed description whentaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a schematic view of a re-configurable optical add-dropmultiplexer (OADM) in accordance with a first embodiment of the presentinvention;

[0012]FIG. 2 is a schematic view of a re-configurable OADM in accordancewith a second embodiment of the present invention;

[0013]FIG. 3 is a side view of a fiber Bragg grating (FBG) of FIG. 1;

[0014]FIG. 4 is a top view of the FBG of FIG. 3;

[0015]FIG. 5 is a cross-sectional view of the FBG of FIG. 3, taken alongline V-V of FIG. 3; and

[0016]FIG. 6 is a schematic diagram showing connections between microheating elements of a resistor of the FBG of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Referring to FIG. 1, an optical wavelength division multiplexing(WDM) signal passes into a re-configurable optical add-drop multiplexer(OADM) 800 in accordance with a first embodiment of the presentinvention. The optical WDM signal comprises a plurality of opticalsignals with different wavelengths from λ₁ to λ_(N), wherein N is aninteger higher than 1. The re-configurable OADM 800 comprises a firstoptical circulator 10, a second optical circulator 30 and a fiber Bragggrating (FBG) 20.

[0018] The first optical circulator 10 further comprises a firstcirculator port 11, a second circulator port 12 and a third circulatorport 13. An optical signal passing into the first circulator port 11 isrouted to the second circulator port 12, and an optical signal passinginto the second circulator port 12 is routed to the third circulatorport 13. Similarly, the second optical circulator 30 also comprises afirst circulator port 31, a second circulator port 32 and a thirdcirculator port 33. An optical signal passing into the first circulatorport 31 is routed to the second circulator port 32, and an opticalsignal passing into the second circulator port 32 is routed to the thirdcirculator port 33.

[0019] Referring to FIGS. 3-6, the FBG 20 includes a cylindrical opticalfiber 22 forming an optical fiber core 220 therein, a thermal electriccooler (TEC) 23 and a resistor 24. Instead of utilizing thephotosensitivity of different optical materials, the FBG 20 takesadvantage of the fact that the index of refraction (n) of some opticalmaterials varies with temperature. Both silica-based materials and manydifferent optical polymers demonstrate such a relationship. Forinstance, the index of refraction, n, of silica increases withincreasing temperature.

[0020] The TEC 23 is mounted on one side of one section of the opticalfiber 22, which side has been polished into a flat surface. The TEC 23makes a good physical contact with the flat surface. A thin layer ofthermal epoxy can be applied to the flat surface to ensure good thermalconductivity between the optical fiber 22 and the TEC 23. The TEC 23acts as a heat sink and as a temperature controller to maintain thesection of the optical fiber 22 at a pre-determined temperature.

[0021] Referring FIG. 4, the resistor 24 includes a plurality of microheating elements 240 and a plurality of bonding wires 242 connecting themicro heating elements 240 together. The micro heating elements 240 aredeposited in an evenly spaced pattern on an outside surface of theoptical fiber 22 by deposition or photolithography, each micro heatingelements 240 having a shape of the letter “C”, with an opening thereoffacing the TEC 23 (best see FIG. 5). Each micro heating element 240 canbe a very thin layer of metal or other material that conducts current.The micro heating elements 240 are connected in series by the bondingwires 242, that is, one end of a micro heating element 240 is wirebonded to a neighboring heating element 240 in front of it by one of thebonding wires 242, while the other end is wire bonded to anotherneighboring heating element 240 behind it by another bonding wire 242(best see FIG. 6). Thus, the same current flows through all the microheating elements 240 in a zigzag fashion. Because the micro heatingelements 240 are resistive, heat will be generated by the micro heatingelements 240 when current flows through them.

[0022] When no current flows through the resistor 24, the section of theoptical fiber 22 stays at one uniform temperature (the same as that ofthe TEC 23), so the index of refraction (n) of the fiber core 220 isuniform, and no Bragg Grating effect will affect light transmittingthrough the optical fiber 22 when it reaches the section with the TEC23. This state is called the “all pass” state.

[0023] When current I_(i) flows through the resistor 24, however, eachmicro heating element 240 of the resistor 24 generates heat at aconstant rate as long as the current I_(i) remains constant. Each microheating element 240 raises the temperature in a cross section of theoptical fiber 22 that sits directly beneath such micro heating element240 and very close to either side of said cross section. The crosssections of the optical fiber 22 which are between the micro heatingelements 240 remain at a temperature approximately the same as that ofthe TEC 23, since the TEC 23 has a much larger contacting area with theoptical fiber 22 than the micro heating elements 240 have, and since theTEC 23 has a large heat transfer capacity. Hence, a series of uniform,evenly distributed “hot spots” develops along the optical fiber 22 whencurrent flows in the micro heating elements 240. Because the index ofrefraction of the materials in the optical fiber core 220 varies withtemperature, a periodic pattern of indexes of refraction varying betweentwo values is generated inside the optical fiber core 220, with an indexof refraction in the “hot spots” being different from that in theunheated areas. This periodic variation in the index of refractionconstitutes a fiber Bragg grating (FBG). A distance between adjacent“hot spots” is the pitch (Λ) of the FBG 20, which determines whatwavelength is most strongly reflected by the FBG 20.

[0024] Different values of current heat the “hot spots” to differenttemperatures, thus changing a difference between the indexes ofrefraction in the heated and in the un-heated areas. Because thereflection ratio of incident light of an FBG is determined by thedifference in indexes of refraction in the “hot” areas and in the “cold”areas, and by the length of the grating, then by choosing thetemperature of the TEC 23, as well as the length of the grating area, wecan find a reasonable driving current (I_(max)) whereat nearly 100%reflection can be achieved for the wavelength that meets the Braggcondition. The state of the FBG at this driving current value is calledthe “all reflect” state. In this state, a decrease in the drivingcurrent will cause that particular wavelength of the incident light tobe partially reflected and partially passed.

[0025] Thus, for a particular wavelength that meets the Bragg condition,the FBG 20 can achieve the “all reflect” (100% reflection) state whenI_(i)=I_(max), the “all pass” state (100% pass) when I_(i)=0, and a“partial reflection” (thus partially pass state) when 0<I_(i)<I_(max),by choosing different driving currents. Therefore, the FBG 20 functionsas a dynamic fiber Bragg grating.

[0026] Referring again to FIG. 1, the FBG 20 is disposed between andconnected with the second circulator port 12 of the first opticalcirculator 10 and the second circulator port 32 of the second opticalcirculator 30. The FBG 20 can reflect an optical signal with aparticular wavelength λ_(i), according to the distance between theadjacent micro heating elements 240 (best see FIG. 3), wherein thewavelength λ_(i) is among the wavelengths from λ₁ to λ_(N) of theoptical WDM signal.

[0027] The operation of the re-configurable OADM 800 is described asfollow:

[0028] When the FBG 20 is put in the “all reflect” state, the FBG 20reflects an optical signal with the wavelength λ_(i), and there-configurable OADM 800 functions to add-drop the optical signal withthe wavelength λ_(i) from the optical WDM signal. The optical WDM signalenters the re-configurable OADM 800 from the first circulator port 11.The optical WDM signal is routed from the first circulator port 11 tothe second circulator port 12, and passes into the FBG 20. The opticalsignal with the wavelength λ_(i) is reflected back to the secondcirculator port 12 by the FBG 20, and is routed from the secondcirculator port 12 to the third circulator port 13. Thus, the opticalsignal with the wavelength λ_(i) is dropped from the WDM signal. Theremaining optical signals with the wavelengths different from thewavelength λ_(i) pass through the FBG 20 and into the second circulatorport 32 of the second optical circulator 30. The remaining opticalsignals are routed from the second circulator port 32 to the thirdcirculator port 33, thus being output from the re-configurable OADM 800.In the meantime, a new optical signal can be added by entering the firstcirculator port 31, wherein the new optical signal carries newinformation and has the same wavelength λ_(i). The new optical signal isrouted from the first circulator port 31 to the second circulator port32, and is also reflected back to the second circulator port 32 by theFBG 20. The new optical signal is then routed from the second circulatorport 32 to the third circulator port 33, and is output from there-configurable OADM 800. Thus, the new optical signal joins theremaining optical signals at the third circulator port 33 to forming anewly-created optical WDM signal.

[0029] When the FBG 20 is put in the “all pass” state, optical signalspass through the FBG 20, no matter what wavelengths they have. Theoptical WDM signal passes through the first circulator port 11, thesecond circulator port 12, the FBG 20 and the second circulator port 32in turn, and is then output from the third circulator port 33. Nooptical signal is dropped or added in the re-configurable OADM 800.

[0030] Therefore, an optical signal with the wavelength λ_(i) isselectively dropped or not dropped from the optical WDM signal byputting the FBG 20 in the “all reflective” or “all pass” state, whereinthe state of the FBG 20 is under the control of the current I_(i)flowing through the resistor 24 (best see FIG. 4).

[0031] Referring to FIG. 2, an optical WDM signal passes into are-configurable OADM 900 in accordance with a second embodiment of thepresent invention. The optical WDM signal comprises a plurality ofoptical signals with different wavelengths from λ₁ to λ_(N), wherein Nis an integer higher than 1. The re-configurable OADM 900 comprises afirst optical circulator 10, a second optical circulator 30 and aplurality of FBGs from 40(#1) to 40(#M), wherein 1<M≦N.

[0032] Comprehensibly, the first and second optical circulator 10, 30have the same structure described in the first embodiment, and each FBG40 also has the same structure as the FBG described in the firstembodiment. The FBGs from 40(#1) to 40(#M) are connected in series andare disposed between the first optical circulator 10 and second opticalcirculator 30. The first FBG 40(#1) is connected with the secondcirculator port 12 of the first optical circulator 10, and the last FBG40(#M) is connected with the second circulator port 32 of the secondoptical circulator 30. Each FBG 40 can reflect an optical signal with aparticular wavelength, and the FBGs taken together, from 40(#1) to40(#M), can reflect optical signals having different wavelengths fromλ_(i), to λ_(k), respectively, wherein the wavelengths from λ_(i) toλ_(k) are among the wavelengths from λ₁ to λ_(N) of the optical WDMsignal.

[0033] The operation of the re-configurable OADM 900 is substantiallythe same as the operation described in the first embodiment. The onlydifference is that more than one FBGs can be put in its corresponding“all reflect” or “all pass” states respectively in the secondembodiment.

[0034] The optical WDM signal enters the first circulator port 11 of thefirst optical circulator 10, and is routed to the second circulator port12. When some of the FBGs from 40(#1) to 40(#M) are selected to be putin their “all reflect” states and the rest of the FBGs from 40(#1) to40(#M) are put in their “all pass” states, the optical signals havingthe same wavelengths as the given wavelengths of the selected FBGs 40are reflected by the selected FBGs 40 and dropped from the thirdcirculator port 13 of the first optical circulator 10. The “givenwavelength” above is defined here as the wavelength reflected by asubject FBG when it is in its “all reflect” state. The remaining opticalsignals pass through the FBGs from 40(#1) to 40(#M) in turn, and arerouted from the second circulator port 32 to the third circulator port33. In the meantime, new optical signals having the same wavelengths asthe dropped optical signals but carrying new information can be added byentering the first circulator port 31. The new optical signals are alsoreflected by the selected FBGs 40 and are output from the thirdcirculator port 33, and join the remaining optical signals to forming anewly-created optical WDM signal. The number of the selected FBGs ischangeable, and the number of the wavelengths of the dropped opticalsignals is likewise changeable. Therefore, the re-configurable OADM 900can add and drop the desired optical signals from the optical WDM signalby putting the corresponding FBGs 40 in the “all reflect” states and therest of the FBGs 40 in the “all pass” states, wherein the states of theFBGs from 40(#1) to 40(#M) are under the control of the currents fromI_(i) to I_(k) flowing through the corresponding resistors 24 (bestindicated in FIG. 2 and FIG. 4), respectively.

[0035] It is to be understood, however, that even though numerouscharacteristics and advantages of the present invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, the disclosure is illustrativeonly, and changes may be made in detail, especially in matters of shape,size, and arrangement of parts within the principles of the invention tothe full extent indicated by the broad general meaning of the terms inwhich the appended claims are expressed.

I claim:
 1. A re-configurable optical add-drop multiplexer (OADM)adapted to receiving an optical wavelength division multiplexing (WDM)signal comprising: a first optical circulator having a first circulatorport, a second circulator port and a third circulator port, wherein anoptical signal passing into the first circulator port is routed to thesecond circulator port and an optical signal passing into the secondcirculator port is routed to the third circulator port; a second opticalcirculator having a first port, a second port and a third port, whereinan optical signal passing into the first port is routed to the secondport and an optical signal passing into the second port is routed to thethird port; a fiber Bragg grating (FBG) having an optical fiber, acooler and a plurality of heating elements, the heating elements beingevenly spaced a fixed distance apart and being arranged on the opticalfiber, the optical fiber having an optical fiber core made of a materialwhose index of refraction varies with temperature, the cooler beingmounted on the optical fiber for maintaining a plurality of sectionsbetween pairs of adjacent heating elements of the optical fiber at apre-determined temperature, the FBG having a first state in which itreflects an optical signal with a determined wavelength and passes alloptical signals having other than the determined wavelength and a secondstate in which all the optical signals pass through it; wherein the FBGis connected with the second circulator port of the first opticalcirculator and the second port of the second optical circulator, and theFBG is in the first state selectively for adding an optical signal inthe optical WDM signal through the first port of the second opticalcirculator and for dropping an optical signal from the optical WDMsignal through the third circulator port of the first opticalcirculator.
 2. The re-configurable OADM as described in claim 1, whereinthe wavelength of the optical signal reflected by the FBG is determinedby the distance between the heating elements.
 3. The re-configurableOADM as described in claim 1, wherein the heating elements generate heatwhen current flows through them.
 4. The re-configurable OADM asdescribed in claim 1, wherein the state of the FBG is under the controlof current flowing through the heating elements.
 5. The re-configurableOADM as described in claim 1, wherein the heating elements are depositedin a periodic pattern along an outside surface of the optical fiber. 6.The re-configurable OADM as described in claim 5, wherein the index ofrefraction of the cross section of the optical fiber core directlybeneath each heating element varies with the current flowing through theheating element.
 7. The re-configurable OADM as described in claim 1,wherein the optical WDM signal is input into the re-configurable OADMthrough the first circulator port of the first optical circulator and isoutput from the re-configurable OADM through the third port of thesecond optical circulator.
 8. A re-configurable optical add-dropmultiplexer (OADM) adapted to receiving an optical wavelength divisionmultiplexing (WDM) signal comprising: a first optical circulator havinga first circulator port, a second circulator port and a third circulatorport, wherein an optical signal passing into the first circulator portis routed to the second circulator port and an optical signal passinginto the second circulator port is routed to the third circulator port;a second optical circulator having a first port, a second port and athird port, wherein an optical signal passing into the first port isrouted to the second port and an optical signal passing into the secondport is routed to the third port; a plurality of fiber Bragg gratings(FBGs), each FBG having an optical fiber, a cooler and a plurality ofheating elements, the optical fiber having an optical fiber core beingmade of a material, wherein an index of refraction of the materialvaries with temperature, the heating elements being evenly spaced afixed distance apart and being arranged on the optical fiber, the coolerbeing mounted on the optical fiber for maintaining a plurality ofsections between pairs of adjacent heating elements of the optical fiberat a pre-determined temperature, each FBG having a first state in whichit reflects an optical signal with a given wavelength and pass opticalsignals having other than the given wavelength and a second state inwhich all the signals pass through it; wherein the FBGs are connected inseries and are positioned between the first optical circulator and thesecond optical circulator, the first FBG is connected with the secondcirculator port of the first optical circulator and the last FBG isconnected with the second port of the second optical circulator, andeach FBG is in the corresponding first state selectively for addingoptical signals into the first port of the second optical circulator andfor dropping optical signals from the third circulator port of the firstoptical circulator.
 9. The re-configurable OADM as described in claim 8,wherein the wavelength of the optical signal reflected by each FBG isdetermined by the distance between the corresponding heating elements.10. The re-configurable OADM as described in claim 8, wherein theheating elements generate heat when current flows through them.
 11. There-configurable OADM as described in claim 8, wherein the state of eachFBG is under the control of current flowing through the heatingelements.
 12. The re-configurable OADM as described in claim 8, whereinthe heating elements are deposited in a periodic pattern along anoutside surface of the corresponding optical fiber.
 13. There-configurable OADM as described in claim 12, wherein the index ofrefraction of the cross section of the optical fiber core directlybeneath each heating element varies with the current flowing through theheating element.
 14. The re-configurable OADM as described in claim 8,wherein the optical WDM signal is input into the re-configurable OADMthrough the first circulator port of the first optical circulator and isoutput from the re-configurable OADM through the third port of thesecond optical circulator.
 15. The re-configurable OADM as described inclaim 8, wherein the re-configurable OADM puts the desired FBGs in thefirst states and the rest of the FBGs in the “all pass” states to dropthe optical signals having the same wavelengths as the given wavelengthsof the desired FBGs from the optical WDM signal.
 16. An arrangement of are-configurable optical add-drop multiplexer and light used therewith,comprising: a first optical circulator defining a first circulator port,a second circulator port and a third circulator port, wherein an opticalsignal passing into the first circulator port is routed to the secondcirculator port and an optical signal passing into the second circulatorport is routed to the third circulator port; a second optical circulatordefining a first port, a second port and a third port, wherein anoptical signal passing into the first port is routed to the second portand an optical signal passing into the second port is routed into thethird port; and a plurality of reflectors, for respectively reflectingsignals of different wavelengths, being connected between the secondcirculator port and the second port; wherein light including pluralsignals and coming from the first circulator port, leaves from the thirdport, with associatively at least some new signals having selectivedifferent wavelengths and coming from the first port and reflected by atleast some selected working ones of said reflectors, and without atleast some old signals having said selective different wavelengths andcoming along with said light from the first circulator port whilereflected by said at least some selected working ones of said reflectorsand leaving from the third circulator port.
 17. The arrangement asdescribed in claim 16, wherein said reflectors are arranged in series.18. The arrangement as described in claim 16, wherein the rest of saidreflectors are not working so that other signals, which come from thefirst port and are able to be reflected by said rest of said reflectorsif said rest of the reflectors are workable, leave from the thirdcirculator port.
 19. The arrangement as described n claim 18, whereinsome signals, which come from the first circulator port and are able tobe reflected by said rest of the reflectors if said rest of thereflector are workable, leave from the third port.
 20. The arrangementas described in claim 16, wherein said reflectors are fiber Bragggratings (FBGs).
 21. The arrangement as described in claim 20, whereinsaid FBGs are variable so as to decide which of said reflectors are inworking or non-working manner.
 22. An arrangement of a re-configurableoptical add-drop multiplexer and light used therewith, comprising: afirst optical circulator defining a first circulator port, a secondcirculator port and a third circulator port, wherein an optical signalpassing into the first circulator port is routed to the secondcirculator port and an optical signal passing into the second circulatorport is routed to the third circulator port; a second optical circulatordefining a first port, a second port and a third port, wherein anoptical signal passing into the first port is routed to the second portand an optical signal passing into the second port is routed into thethird port; and a reflector, for respectively reflecting a signal of awavelength, being connected between the second circulator port and thesecond port; wherein light including plural signals and coming from thefirst circulator port, leaves from the third port, with associatively atleast a new signal having said wavelength and coming from the first portand reflected by said reflector if said reflector is in a workingmanner, and without at least an old signal having said wavelength andcoming along with said light from the first circulator port whilereflected by said reflector if said reflector is in the working mannerand leaving from the third circulator port; oppositely, if saidreflector is in a non-working manner, said new signal leaves from saidthird circulator port and said old signal leaves from said third port.